I do love the Elma selector switches that Arn sells.
For level match (in SSHD), I latch M/1 to M/2 and pull them down, as a linked pair, for digital attenuation of the monitor DAC - post DAW-reinput, and leave M3/M4 at Unity. At double-precise 24 bit integer, it's no twin-SHARC bus, like UltraInteractive uses, but at least, this way, the eq/comp improvement has to stand up to added quantization distortion (however subtle and/or canceled out by temporarily-inserted 16 bit dither).
I like the purity of passive attenuation, but for trimming, unless you use amps for buffering, which can add coloration, you need to be very careful about the parallel addition of impedance.
When calculating the desired net impedance, I believe that one needs to take into account the input Z of each passive device in circuit, including that of the power amp. Also, one needs to put a numerary "1" above the series of reciprocal values of Z.
_______1_________
1/Za + 1/Zb + 1/Zc
Find out what the optimal load is for your DAC and tune the chain to match. It can be daunting to derive a combination of impedances that will work together, without changing the response, while you are trimming the voltage swings. In the days of power matching (in the studio), the impedances were all symmetrical, but the build up of transformer distortions was one of the price tags for ease of interconnection. Again, using amps, be they transistor-based, or valve, or IC, can add color that might possibly skew the level-matched comparisons, since, once the trimming is back out of circuit, the level is restored to master Unity, but whatever (masking or skewing) coloration had been added is also now eliminated.
How transparent are the amps in the Crookwood? Probably very much so. But, if a Charter Oak falls in the Crookwood, and it is level-matched to source, will anyone really hear what's happened? Or is it yet another perspective?
The source, when cranked up, is always going to have some extra kind of live-ness that limiting will detract somewhat from. So, if the source is too quiet for the client, start lifting... and maybe don't look back?
Andrew